Incident Description
A factory-installed map light and its switch were mounted midway down the left doorpost. Approximately half a mile from the airport, a passenger used the left forward doorpost for support and a pop noise was heard. Flames immediately shot from both sides of the plastic doorpost cover at the level of the map light, and black smoke filled the cockpit. The pilot attempted to smother the fire, but each time the flames returned and intensified. An emergency downwind landing was made on runway 1 as the flames completely involved the left side of the cockpit. The cabin was destroyed in the subsequent ground fire.
Investigation Findings
It was unclear if Airworthiness Directive (AD) 80-04-08, issued on February 16, 1980, was complied with. The AD addressed instances of fuel leaks and/or in-flight fires resulting from contact between the map light switch terminals and an adjacent fuel line. It required a one-time visual inspection of the left doorpost area to ensure at least 0.50-inch separation between the switch and the fuel line, and installation of an insulator cover over the map switch electrical terminals.
A mechanic had repaired the map light the day before the accident due to a pilot squawk. He found a loose wire and reconnected it to the switch. In initial questioning, the mechanic stated there was nothing between the switch and the fuel line; in a second interview, after reviewing the AD and parts manual diagrams, he stated he definitely reinstalled the insulator cover over the switch terminals.
Review of the 172's design history revealed that models prior to the 172N had one fuel supply line from each tank routed down the aft doorpost, with map lights located at the top of the doorpost where there was more internal clearance. With the 172N model change, a second fuel feed line was added from each tank and routed down the forward doorpost in the channel with wire bundles and the map light switch, with nylon spacers to separate wires from the fuel line. The map light and switch were moved to their present location. The only flammable components in the doorpost are wire insulation, nylon spacers, plastic doorpost covering, and fuel in the fuel line (0.75 psi head pressure at map light level).
No evidence of electrical shorts in doorpost circuits or flammable fluid leaks external to the doorpost that would account for the ignition and severity of the fire was found. The NTSB accident records contained three accidents where fires originated in the left forward doorpost; all three aircraft were Cessna 172Ns. The SDR database had three instances (all 172Ns) where electrical arcs from the map light switch punctured the fuel line, causing in-flight fuel leaks. Other SDRs documented over 12 instances of electrical wires chafing into fuel lines in either forward doorpost. Examination of two exemplar Cessna 172Ns revealed that internal configuration of doorpost components did not conform to original drawing specifications due to interior replacements and repairs over the years.